This invention relates to a stepless speed change electric chain block capable of changing lifting and lowering speeds in stepless manner.
The assignee of this case has proposed a stepless speed change electric chain block (Japanese Laid-open Patent Application No. 55-156,194). The proposed stepless speed change electric chain block is of small size and light weight and economical in use because stepless speed change of a load sheave is accomplished by the use of an AC motor without using a DC power source. Moreover, the proposed stepless speed change electric chain block comprises screw thread type mechanical brake means provided in a transmission mechanism between the AC motor and the load sheave for automatically preventing the load sheave from being rotated in a winding-off direction due to a load. The load sheave is therefore prevented from being rotated in the lowering direction at higher speeds than those of a rotor of the motor, no matter how large the torque due to the load. Accordingly, the proposed electric chain block can carry out the lowering operation at safe and stable speeds.
On the other hand, however, the proposed electric chain block requires a tachometer for detecting speeds and a voltage comparison circuit for setting the speed of the load sheave, whose control system becomes unavoidably complicated.